Autochthonous organic carbon

Stable carbon isotopes have been commonly used to distinguish between allochthonous versus autochthonous organic carbon inputs to estuaries. One of the most important pieces of information that can be gathered from this information is the delineation between C3 and C4 plant inputs. 

Figure 1. Allochthonous and autochthonous sources of dissolved organic carbon in natural waters. (Reproduced with permission from reference 1. Copyright 1985 Kluwer Academic Publishers.)...

The formula for BCD in Eq. (2) was derived under the assumption that the bacterial growth rate was mineral nutrient limited. If the supply rate of labile organic carbon from allochthonous and autochthonous sources is insufficient to meet this demand, the pool of labile dissolved organic carbon (DOC) will eventually be depleted and the bacteria will become carbon... 

In lakes, the pool of dissolved organic carbon (DOC) is dominated by dissolved humic substances (up to 80% of the DOC). Lake humic substances are similar to soil humic substances in that carboxyl, hydroxyl, phenol, and probably methoxyl groups are of major significance. Fluorescence spectra of DOC may be interpreted in terms of the different geochemical origins of DOC (e.g., allochthonous versus algal derived). One or more moieties of dissolved humic substances are produced autochthonously mechanisms may include polymerization of phenols (promoted by transition metals), Maillard condensations, or oxidation via phenolase systems. Aliphatic structural units in dissolved humic substances provide a flexible conformation to the humic substance molecule. ... 

We hypothesized that the input of terrestrial organic matter relative to the input of autochthonous carbon would be a good predictor of the relative importance of heterotrophic organic N formation in different aquatic systems. Following a flow path from streams to lakes to rivers to estuaries to oceans, this hypothesis would suggest that microbial organic N formation would be highest in wooded streams and rivers, moderate in lakes and estuaries, and lowest in open-water marine systems. 

The DOC/enzyme/microbe interaction (DEMI) model divides bacterioplankton into two functional guilds, opportunists and decomposers, and DOC into two pools, labile and recalcitrant. In the context of the model, labile DOC is defined as directly assimilable monomers (saccharides, amino acids, and organic acids) and readily hydrolyzed polymers (polysaccharides, proteins, and nucleic acids). Because these substrates turn over rapidly, thus are unlikely to be transported far, most of the carbon in this pool will be autochthonous lysates and exudates, or allochthonous leachates from storms or seasonal litter fall. Recalcitrant DOC is defined as humic substances created by oxidative reactions among proteins, polysaccharides, hydrocarbons, and phenolic molecules. For inland waters, recalcitrant DOC is largely of allochthonous origin. 

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